STRUCTURAL GEOLOGY

Six attempts were made to spud holes in the Snowcap hydrothermal site, but core was recovered from only two holes (Holes 1188A and 1188F). Hole 1188A represents the shallow part of the hydrothermal system under the vent field, from 0 to 211.6 mbsf, whereas Hole 1188F represents the deeper part of the system, from 218.00 to 378.50 mbsf. The distance between the holes is only ~30 m.

The structures identified in these cores were primary volcanic layering, vein orientations, and vein relationships. We also recorded the mineralogy of the veins and, where present, the extent, intensity, and mineralogy of alteration halos around the veins. The observations were described on the structural description forms, and we entered the data in the structural log (see "Site 1188 Structural Geology Descriptions" and "Site 1188 Structural Log").

Hole 1188A

Orientation of Primary Layering

In the volcanic rocks, original layering was identified from the orientation of elongate flattened and stretched vesicles in some of the massive lavas and, in other parts of the core, from millimeter-scale flow banding. Individual flow bands may be defined by differences in the abundance and size of the spherulites or microlites formed during crystallization or by differing degrees of vesicularity. This banding has commonly survived the extensive hydrothermal alteration.

Seven pieces of core contained evidence for folding (intervals 193-1188A-8R-1 [Piece 8, 65-74 cm], 12R-2 [Piece 4, 36-52 cm], 16R-1 [Piece 10, 69-79 cm], 16R-2 [Piece 3, 16-20 cm], 16R-2 [Piece 6, 31-39 cm], and 16R-2 [Piece 12, 86-94 cm]), which is defined by millimeter-scale flow banding (Fig. F97). Both upright folding and recumbent, probably intraformational folding, are evident. Boudinlike disruption of the original layering is clearly evident in interval 193-1188A-16R-1 (Piece 14, 95-102 cm) (Fig. F98). The folding and disruption of layering are most likely to have been caused by deformation during the flow of the lava, indicating a relatively viscous magma.

Twenty-five dip measurements of the volcanic layering were made on oriented pieces of core (see "Structural Geology" in the "Explanatory Notes" chapter). Throughout the core, the dips of the layering vary from horizontal to very steep (Fig. F99). These variations in dips could be caused by the intersection of several detached and rotated lava blocks and/or intersections of proximal to distal parts of lava flows with respect to the extrusion sites.

Vein Descriptions

The veins in the core from Hole 1188A predominantly consist of varying proportions of anhydrite, silica minerals (cristobalite and quartz), clay minerals, pyrite, and magnetite (Fig. F100A). Chalcopyrite and sphalerite are present in trace amounts in some of the veins. According to their mineralogy and crosscutting relationships, the veins are broadly categorized as one of three types: (1) anhydrite ± pyrite veins, (2) anhydrite ± pyrite ± silica ± magnetite veins, and (3) anhydrite + silica + clay ± pyrite veins. The most common veins are those with anhydrite with or without pyrite, silica, or magnetite; these veins are present in most of the core. The anhydrite + silica + clay ± pyrite veins form a dense network, restricted to lithologic Units 10 and 11 in the curated interval at 97.38-107.39 mbsf (interval 193-1188A-12R-1, 78-91 cm, to 13R-1, 97-109 cm).

Anhydrite ± Pyrite ± Silica ± Magnetite Veins

Anhydrite ± pyrite ± silica ± magnetite veins are present between Unit 4 (i.e., Section 193-1188A-7R-1, 31 cm; 48.51 mbsf) and the bottom of the core (Fig. F100A). The veins in the upper part of the core between Units 1 and 15 (i.e., above Section 193-1188A-15R-1; 125.71 mbsf) are dominated by anhydrite or cristobalite, whereas veins dominated by quartz are common in the interval between Units 15 and 25 (i.e., between Sections 193-1188A-15R-1 [126.51 mbsf] and 21R-1 [184.05 mbsf]). Magnetite is a minor to major component of the veins in lithologic Units 19-25 (i.e., between Sections 193-1188A-17R-1 [146.17 mbsf] and 21R-1 [184.05 mbsf]). These veins were plotted separately in Figure F100A, as silica-pyrite, pyrite, or magnetite-pyrite-silica-anhydrite veins.

The veins are present as single veins, branching veins, arrays of multiple veins, and as vein networks. The thickness of these veins ranges from <0.5 mm to >28 cm (see Fig. F100B). Their dips range from horizontal to vertical (Fig. F100C). The veins in volcanic rocks with well-preserved perlitic textures tend to follow the primary perlitic fracturing. Furthermore, in those rocks with primary flow banding, minor veins and veinlets tend to parallel individual bands, occasionally being interlinked by thicker crosscutting veins (Fig. F101A). Only one example of a vein showing multigenerational crack-seal opening was found. In this vein, two stages of anhydrite veining were recorded, both with cristobalite halos (Fig. F101B).

Most of the veins are surrounded by alteration halos of 1 mm to several centimeters thickness (Fig. F102). Soft, white halos of a clay mineral, tentatively identified as pyrophyllite, are typical around the veins in Unit 9 between 87.05 and 96.71 mbsf (i.e., Sections 193-1188A-11R-1, 15 cm, to 12R-1, 11 cm), and again around veins in Unit 11 (i.e., interval 193-1188A-13R-1, 59-138 cm [106.89-107.68 mbsf]) (Fig. F103). Silica halos are common around the veins in Units 4-8 (i.e., Sections 193-1188A-7R-1 to 9R-1) and in the deeper part of the core (i.e., from Section 193-1188A-14R-1 [116 mbsf]). In intervals of network veining, as in Unit 6 between 58.74 and 59.18 mbsf (i.e., interval 193-1188A-8R-1, 97-140 cm), silicification is pervasive around the cristobalite-pyrite bearing veins and affects the entire rock; nevertheless, the original texture of the volcanic rock is still recognizable (see Fig. F49). The rock was strongly fragmented during the stage of silica veining (Fig. F101C), which produced cristobalite in the open spaces between the rock fragments.

The anhydrite in the veins varies from very fine grained and milky-white sugary to coarse-grained with 1- to 2-mm colorless crystals. In some cases the anhydrite is present as fibrous crystals aligned perpendicular to the vein walls, suggesting growth in open fractures. Pyrite is present generally as euhedral cubes or pyritohedra of 0.1-0.5 mm size. Silica minerals are present mainly as fine-grained fibrous crystals of cristobalite or as anhedral, more coarse grains of quartz.

In most of the veins, as examined under the microscope, anhydrite is found in the center of the veins, whereas pyrite is in the bands along the rims of the veins. Silica (mainly cristobalite, according to XRD analysis) commonly forms selvages and alteration halos around the veins. Fine-grained pyrite defines the outermost part of these halos. Thus, pyrite is present both as coarse grains in the veins and as fine grains in the silica halos in the country rock around the veins (Fig. F101D). In thin sections from Unit 10 (i.e., Section 193-1188A-12R-1), there are highly irregular and anastomosing veins and veinlets of quartz with scattered grains of anhydrite and minor pyrite (Fig. F101E). These veins are rimmed by a brownish silica-clay halo, followed by a hematite-rutile rim with minor pyrite in the country rock farther out from the veins.

The magnetite-bearing veins in Unit 19 are found below 146 mbsf (Section 193-1188A-17R-1 and downward) and consist of coarse anhydrite (as thick as 1 mm) with scattered magnetite and pyrite. In the open vugs and the thicker veins, the anhydrite tends to be acicular, suggesting growth in a preexisting opening. Alteration halos of bleaching, which probably consist of quartz and clay minerals, extend from the veins for 5-20 mm.

Anhydrite ± Pyrite Veins

Late, thin (1 mm in thickness) anhydrite veins with or without trace pyrite cut the anhydrite ± pyrite ± silica ± magnetite veins that are surrounded by halos of alteration (Fig. F101F). These later anhydrite ± pyrite veins are characterized by having no alteration halos and the anhydrite tends to be coarse grained.

Anhydrite + Silica + Clay ± Pyrite Veins

Veins of anhydrite + silica + clay are present between Unit 10 starting at 97.38 mbsf (i.e., interval 193-1188A-12R-1, 78-91 cm) and Unit 11 at 107.39 mbsf (i.e., interval 193-1188A-13R-1, 97-109 cm). The proportions of the constituent minerals vary from being predominantly anhydrite + clay minerals to predominantly silica + clay minerals. These veins form a dense network of veins <0.5-1 mm wide, overprinting earlier alteration assemblages of gray to greenish chlorite + clay minerals (Unit 10; Sections 193-1188A-12R-1 and 12R-2) (Fig. F102) and gray to white silica + anhydrite + clay (Unit 11; Section 193-1188A-13R-1) (Fig. F103). In these veins, the grain size is <0.1 mm. The network of veins is cut by anhydrite ± pyrite veins, which are especially common in Unit 11 (i.e., Section 193-1188A-13R-1) (Fig. F103).

Vein Geometries

Veins are common in Unit 4 from 48.51 mbsf (i.e., interval 193-1188A-7R-1, 31-42 cm) to the bottom of the core. The sections above Unit 4 are fresh to completely altered volcanic rocks characterized by moderate to pervasive alteration containing only very fine silica veinlets (see "Hydrothermal Alteration"). The veins vary in thickness from hairline-thick veinlets (<0.1 mm thick) to veins thicker than 28 cm. Overall, 76% of the veins are 1 mm or less in thickness, whereas only ~6% of the veins are thicker than 1 cm (Fig. F100B). The thickest vein was found in Unit 15 and the upper part of Unit 16 (i.e., interval 193-1188A-15R-1, 0-28 cm [125.7-125.98 mbsf]), in the form of six smaller pieces of coarse anhydrite (0.1-0.5 mm grains) with minor pyrite.

The orientations of the veins, measured relative to the core axis in the oriented pieces, show that the dips of veins range from horizontal to vertical with a tendency for steeper dips in the lower part of the hole (Fig. F100C).

Vein Parageneses

The following vein parageneses are established, from oldest to youngest:

  1. Anhydrite + silica + clay veins,
  2. Anhydrite ± pyrite ± silica ± magnetite veins, and
  3. Anhydrite ± pyrite veins.

The earliest veins formed as dense networks of anhydrite, silica, and clay minerals and postdate pervasive grayish green GSC alteration in the volcanic rocks (Figs. F102, F103). In places (despite this alteration) the original flow banding/lamination of the protolith has been preserved (Fig. F97). The network of anhydrite + silica + clay veins is cut by anhydrite + silica + pyrite veins and is overprinted by their coeval alteration halos (Figs. F102, F103). The anhydrite ± pyrite ± silica ± magnetite veins are characterized by having alteration halos of clay minerals or silica. These veins and their halos are cut by later anhydrite veins without halos.

Hole 1188F

The ADCB cores from Hole 1188F were highly fragmented, and only a few structural features could be measured. The measurements were too few to determine the general attitude of primary volcanic layering. In a few cases, structural measurements were made before the cores were split, because the high degree of core fragmentation made it impossible to preserve the structures after curation. This was especially the case for vein structures.

Primary Volcanic Structures

We observed very few primary volcanic structures in Hole 1188F. We noted alternating light gray to whitish bands in a few cases, probably representing flow banding. In other cases, we were able to define the primary layering from the orientation of flattened and stretched vesicles, typically filled by pyrite and/or anhydrite. Aligned laths of plagioclase also defined volcanic layering in some samples. The few data obtained (not plotted) (see the "Site 1188 Structural Log") show dips ranging from subhorizontal to subvertical. As in Hole 1188A (see above), this variation can be explained by the intersection of different parts of flows, or the intersection of rotated and faulted lava blocks.

Vein Mineralogy

The veins in Hole 1188F consist predominantly of varying proportions of silica, anhydrite, pyrite, and magnetite (Fig. F104). About 57% of the veins are anhydrite-pyrite veins, and if the anhydrite-pyrite veins containing silica are included, this percentage rises above 70%. Approximately 11% of the veins are silica-pyrite veins, 12% are veins with only pyrite, and the remaining 7% of the veins contain magnetite with a variable content of pyrite, minor quartz, and/or anhydrite. With respect to mineralogy of the veins with depth in the hole, the amount of silica in the veins decreases downward in the hole until Unit 63, where it increases again and is present in the magnetite-bearing veins (Fig. F105). The shallowest magnetite-bearing vein is in Unit 52 (i.e., Section 193-1188F-31Z-1), and magnetite-bearing veins increase in abundance downhole. Possible marcasite was observed in a few pieces from Unit 38 (Section 193-1188F-11G-1), whereas euhedral crystals of light yellow to brown sphalerite were found associated with pyrite in veins of coarse anhydrite in Units 66 and 70 (i.e., Sections 40Z-1 and 42Z-1).

In many cases the anhydrite-pyrite veins are surrounded by silica-clay alteration halos, which, around the thicker veins (1 mm), are commonly cyclic and consist of alternating millimeter-thick layers colored different shades of gray (Fig. F106). Examination under the microscope showed that the differences between these layers are caused by variations in the proportions of clay minerals to very fine grained quartz (Fig. F107). The silica-pyrite, pyrite, and magnetite-bearing veins are usually <0.5 mm thick (hairline) and have either bleached halos of white clay minerals and minor silica or no halos.

Vein Parageneses

The vein parageneses in Hole 1188F are very complex and result from several episodes of fluid infiltration. This is evidenced by the cyclic nature of the alteration halos around many of the veins and by the various crosscutting relationships. However, in many cases the crosscutting veins have the same mineralogy and are surrounded by the same type of alteration halos, showing that the fluids forming the different veins had generally the same composition. This suggests that the fluids were flushed episodically through the rock during a longer period of continuing hydrothermal activity, rather than during one distinct event.

Interval 193-1188F-14Z-1 (Piece 6, 96-108 cm) in Unit 41 is a good example showing the dynamics of vein evolution in Hole 1188F (Fig. F108). The veins in this piece consist predominantly of anhydrite and pyrite. The two thicker veins, labeled Va and Vb, are surrounded by gray, 1- to 2-mm-thick, siliceous halos that grade outward into 1- to 2-mm-thick, light gray halos more rich in pyrophyllite. A thinner siliceous halo is present around vein Vc. In all veins except for the thickest vein (Va) and the thinnest veins (Vj-Vp), pyrite occupies the center of the veins and is rimmed by anhydrite. Vein Va has coarse anhydrite in the center of the vein, rimmed by thin veinlets of pyrite, followed by a thin rim of anhydrite. The thinnest veins are either pyrite veins (Vj-Vm) or veinlets of anhydrite (Vn-Vp).

Some of the thinner veins branch off from the thicker veins (e.g., Vi and Vm from Va, Vk and Vl from Vb, and Vg and Vj from Vc). Veins Vc and Vd are probably part of the same vein, crosscut by Vb. This is evinced by the anhydrite selvages of veins Vc and Vd, which are overprinted by the siliceous halo around Vb (Point A in Fig. F108B). Furthermore, the right-lateral offset between Vc and Vd matches the space of the extensional jog filled with pyrite of Vb (Point B in Fig. F108B). Finally, the anhydrite crystals are aligned east-west in the vein intersection at Point A, indicating an east-west extension. Careful examination shows that Ve cuts across the halos around both Va and Vb, indicating it to be later. Although these veins show crosscutting relationships, on the basis of their mineralogy and the nature of alteration halos, most of the veins in this specimen are considered to be part of the same main veining event, which is responsible for the formation of the abundant late anhydrite-pyrite veins that are characteristic of the cores from Hole 1188F.

The coarse anhydrite in the center of vein Va crystallized in open space, and crystals in vugs within the vein are preferentially aligned vertically, indicating a vertical extension. The Vn, Vo, and Vp veinlets are offshoots from this anhydrite vein, and they cut the pyrite veinlets, the anhydrite layer rimming the central anhydrite, and also the surrounding alteration halos. This shows that the coarse anhydrite veining represents a late-stage event, opening up the preexisting vein Va, which had formed, as had the other veins in the piece, as a pyrite-anhydrite vein with pyrite in the center rimmed by anhydrite. Similar coarse anhydrite veins were encountered in several pieces downhole until Unit 50 (i.e., interval 193-1188F-26Z-1, 31-94 cm). Another good example of late-stage coarse anhydrite veining was observed in interval 193-1188F-25Z-1 (Piece 5, 30-42 cm) (Fig. F109).

Interval 193-1188F-23Z-1 (Piece 3, 14-24 cm) in Unit 48 shows a very good example of a multiple opening of veins (i.e., it contains unequivocal crack-seal veins) (Fig. F110). The two major veins in this piece, Va and Vb, both consist of coarse anhydrite crystallized in previous openings in the rock. They are both surrounded by 4- to 5-mm-thick selvages or halos consisting of alternating silica-clay and anhydrite layers, each layer being 1 mm or less in thickness. These layers are interpreted to represent several crack-seal events, each leading to a pair of silica-clay and anhydrite layers. The last event was the crystallization of the coarse anhydrite occupying the centers of the veins Va and Vb. Hairline veinlets of anhydrite ± pyrite (Vg, Vh, and Vi), rooted in Va and Vb, cut across the earlier couplets of anhydrite and silica-clay zoning. The vein Vc is a branch of Va, and veins Vf and probably Vd are branches of Vb. Both Vc and Vd are surrounded by cyclic selvages like those rimming Va and Vb, showing that these branches also have been opened several times. The vein Ve ends in the selvage around Va, but gets thicker away from Va, which suggests that it belongs to another vein system. Finally, the vein-forming events have led to extensive diffuse bleaching in the rocks beyond the immediate halos and selvages, which overprints preexisting silicification. This is also evident in Unit 41 (i.e., interval 193-1188F-14Z-1 [Piece 3, 96-108 cm]) (Fig. F108).

Another example of crack-seal veining in Unit 42 was observed under the microscope in Sample 193-1188F-15Z-1, 55-58 cm (Fig. F111). Present in this sample is a 1-mm-thick quartz-pyrite vein, consisting of up to 1-mm-sized quartz grains and up to 0.5-mm-sized pyrite grains. The quartz grains show trails of numerous fine fluid and solid inclusions, both of which are subparallel to the vein margins. Some of these trails also cut across the boundaries between adjacent quartz grains. The vein is surrounded by a zoned alteration halo consisting of fine-grained quartz grading outward into a zone of brown clay minerals.

As mentioned above, magnetite is abundant in the veins from Unit 63 (i.e., Section 193-1188F-39Z-1) to Unit 72 at the end of the hole (Fig. F105). There is a piece of core in Unit 72 (i.e., interval 193-1188F-43Z-1, 56-101 cm) that contains a vertically dipping 23-cm-long and a 2- to 5-mm-thick vein consisting of anhydrite, pyrite, magnetite, quartz, and chlorite (Fig. F112). The center of the main vein (<1 mm) is occupied by quartz and clay. The iron minerals are surrounded by a thin selvage of pale chlorite. The whole assemblage is rimmed by a 2- to 4-mm quartz layer. The vein is surrounded by a magnetite-bearing siliceous halo that extends for ~10 mm out from the vein. However, a faint bleaching can be traced for another 20-30 mm out from the vein structure. Late anhydrite and pyrite-anhydrite veins cut across this vein structure at several places.

Vein Geometries

The veins in Hole 1188F are generally very thin, and >80% of the veins are <1 mm thick (Fig. F113A). Only 3.5% of the veins are thicker than 2 mm, and only one anhydrite vein is thicker than 1 cm (Unit 49; interval 193-1188F-25Z-1 [Piece 5, 30-42 cm]) (see Fig. F109). However, it is apparent from the banded halos that were noted on the ends of a large number of core pieces that many anhydrite veins of unknown thickness were not recovered by the drilling.

The orientations of the veins measured in the oriented pieces show that the dips span the range from subhorizontal to vertical (Fig. F113B). However, >42% of the veins have subvertical to vertical dips (75°-90°). There appears to be no systematic variation in dip with depth in the hole (Fig. F113C).

Summary

Holes 1188A and 1188F are only ~30 m apart and, therefore, together represent one vertical section of the hydrothermal system to a depth of 378.50 mbsf below the Snowcap hydrothermal site. There are several interesting things to note with respect to vein features when comparing different parts of the hydrothermal system:

  1. Cristobalite-bearing veins were only encountered in the upper part of the system, that is, above ~126 mbsf.
  2. Anhydrite and pyrite are the most abundant minerals in the veins and are present from 48 mbsf in Hole 1188A to the bottom of Hole 1188F at 378.50 mbsf.
  3. Magnetite-bearing veins are present at two depth intervals—between 146 and 184 mbsf in Hole 1188A and between 322 and 378.50 mbsf in Hole 1188F.
  4. Crack-seal veins and alteration halos showing multiple zonations were only observed below 218 mbsf. Furthermore, the alteration halos around the veins tend to be more intense in Hole 1188F, compared to Hole 1188A.
  5. Late anhydrite veins are more abundant in Hole 1188F than in Hole 1188A.
  6. Brecciation and network veining are only found in Hole 1188A above 110 mbsf, and such structures were not observed in Hole 1188F.
  7. There are no systematic trends with respect to dips of veins with depth.
  8. More than 80% of the veins are thinner than 1 mm, whereas only 4% of the veins are thicker than 1 cm.

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